Children, pregnant and breast-feeding women are being encouraged to dine on more fish but all consumers are being cautioned to avoid certain species -- usually large predators -- that are likely to harbor mercury in their tissues.

Scientists at the U.S. Food and Drug Administration and the Environmental Protection Agency announced proposed federal guidelines on fish consumption, setting a weekly minimum at eight ounces for children, pregnant and breast-feeding women.

But they are also being advised under the pending guidelines to consume no more than 12 ounces, or two to three meals, per week of low mercury-containing seafood. Tuna should be limited to about six ounces weekly, they said during a telephone news briefing Tuesday.

"This is the first time we are specifying a minimum amount as well as a maximum amount [of weekly fish servings]," said Dr. Stephen Ostroff, the FDA's acting chief scientist.

The scientists said fish consumption is important for fetal and childhood brain development and helps provide a boost in IQ.

Meanwhile, a team of Stony Brook University researchers has conducted an investigation into fish consumption habits on Long Island and concluded that consumers should be aware of predator and prey when it comes to fish.

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The species divide, they say, helps determine which fish are most likely to have accumulated appreciable mercury.

The bigger the fish -- marlin, swordfish, shark and some varieties of tuna -- the greater the likelihood of mercury in their tissues.

In their research, the Stony Brook investigators found elevated blood mercury levels were associated with weekly consumption of specific seafoods: tuna steak and fillets as well as sushi-grade tuna.

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Federal guidelines in 2004 specified that pregnant women consume no more than two weekly servings of fish to avoid mercury exposure. The newly proposed guidelines call for at least two servings and as many as three.

A significant percentage of women nationwide have avoided eating fish, Ostroff said, noting they may have been frightened away by recommendations from a decade ago when health authorities emphasized possible mercury exposure.

The mercury concern, he said, has not gone away, but the benefits of fish consumption outweigh the risks if the guidelines are followed.

Mercury -- particularly methylmercury, the form found in fish -- is not to be taken lightly, said Roxanne Karimi of Stony Brook's Consortium for Interdisciplinary Environmental Research.

"It crosses the blood-brain barrier and it crosses the placenta. It leaves the body slowly," Karimi said.

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"It's a neurotoxin," she added, referring to possible brain and neurological damage.

That said, there are fish that can be sustainably harvested and do not pose a danger to human health, she said.

"Eat low on the food chain," said Karimi, referring to the food chain of the seas. Big fish eat smaller ones. And size matters, she said. The larger the fish, the more likely it has accumulated poisons.